Forward solution ambiguity for fMEG model.

Abstract

Accurate knowledge of the electromagnetic model of the fetal brain is required for quantitative fetal MEG (fMEG) signal analysis. The model is defined by the fetal head position, assuming that volume currents are confined to it. Head position can be located by 3D ultrasound. Since the ultrasound is presently not integrated with fMEG, there is a potential for fetal head motion when the mother moves between the two instruments. To interpret the fMEG data without accurate information about the fetal head position, a model search must be undertaken to localize not only the fMEG source within the fetal head, but also the fetal head within the maternal abdomen. Such model search results in multiple solutions with different source amplitudes, but with roughly the same fit errors [Vrba, 2004]. We show that this behaviour is caused by the forward solution ambiguity. For a given measurement, there is a large region within the maternal abdomen within which all fetal head centers produce reasonably good explanation of the observed signal. The source positions within these different fetal heads are different, but the resulting signals are indistinguishable in the presence of even a small noise. Similar ambiguity also exists in adult MEG, where it usually does not pose a problem because the head position is well known.